This invention relates to deflection yokes for television cathode ray tubes.
Both color and monochrome cathode ray tubes employ deflection yokes positioned to horizontally and vertically deflect the electron beam or beams over phosphor covered viewing screens. There are two common types of yoke winding configurations: saddle and toroidal. Frequently, different types of winding configurations are employed for the separate functions of horizontal and vertical deflection. Such an arrangement is called a hybrid yoke. This invention relates specifically to hybrid yokes with toroidally wound vertical deflection coils on a substantially frusto-conic core of magnetically permeable material.
The prior art describes deflection coils of both winding configurations. In the vast consumer-product market, performance and cost to the consumer are of paramount importance, cost readily translates into material, labor and uniformity of product. With respect to the latter, the art has many suggestions for making yokes with consistently repeatable characteristics. U.S. Pat. No. 3,878,490 to Logan describes a toroidal deflection yoke with individual wire wraps being held in place by an adhesive coating on the core. The adhesive resists wire movement, particularly for the first layer and thus attempts to prevent random wire positioning. U.S. Pat. No. 3,758,888 to Kadota employs wire guide rings to maintain wire positions, with partitions separating the various wire sections. U.S. Pat. No. 3,835,426 shows a toroidal deflection yoke having a core with a plurality of built-in grooves for positioning the coil wires.
Automatic machinery for toroidal coil winding is also well known in the art and U.S. Pat. No. 3,799,462 to Fahrback shows a typical device. The machine described is numerically controlled and responds to a punch tape program or a sequence of instructions. It is stated that this enables each turn of the windings to be accurately placed on the core.
The choice of which coil configuration is used is dependent upon many factors, including the type of cathode ray tube display device, the type and amount of correction desired to be incorporated and the drive system for the yoke. It is also well known that many deflection type errors may be minimized by the nature of the electron gun array and phosphor screen pattern of the tube and by tailoring the picture tube screen to a "nominal" yoke with which it will be used.
Recently cathode ray tubes having horizontal "in-line" electron guns and vertically striped screens have become popular, primarily because of the greater ease of obtaining convergence. The limited amount of dynamic convergence required in a well-designed tube of 90.degree. or smaller deflection angle with "in-line" guns can even be taken up in the yoke design. Indeed, today some tubes are sold with full toroid yokes permanently attached. These tubes do not require dynamic convergence correction and both the tube and yoke are replaceable as a unit. Accuracy of wire placement is obtained by using two or less layers of wire, and precision ground cores with yoke forms having plastic wire guides, all of which are very expensive. Further, a toroidally wound horizontal coil is difficult to fully correct for deflection errors. For example, the above-mentioned tube full toroid yoke combination incorporates external top-bottom pincushion correction circuitry, which may readily be incorporated in saddle-wound horizontal coils. By way of comparison, even delta electron gun tubes of small deflection angle invariably need additional dynamic convergence windings for keeping the electron beams in reasonably good convergence throughout the screen area.
The main difficulty with the toroid winding is that of getting precision in a limited winding space. Ideally, each turn should occupy a precise location, and must, therefore, be physically restrained, which greatly reduces the room for the winding and increases the drive requirements. The available drive circuits use SCR devices, which are not only very expensive, but are not as reliable as conventional transistor drive circuits.
Adding more turns to the toroidally wound coil alleviates the drive requirement but destroys the accuracy of placement. Further, if good dynamic convergence is to be built into the yoke, a definite "winding profile" is needed to develop the appropriate deflection field for the proper beam location.
Additionally, the system using plastic liners with built-in wire guides must have a precise relationship between the liner and core. The standard ferromagnetic core material in use today is not sufficiently controllable as to size for this purpose and each core must be ground to proper dimension. This, of course, is very costly.
The yoke should also be rapidly and repeatably producible, that is "windable" on conventional tape controlled winding machines. There are hybrid yokes existent with randomly wound toroidal vertical deflection coils which do not use additional dynamic convergence. The errors introduced by the nonprecision winding of a multilayer toroidal vertical deflection coil is accepted, and a less than optimum display is tolerated.
Applicants' invention comprises a hybrid yoke in which the saddle-wound horizontal deflection coils are accurately aligned with the precision wound toroidal vertical coils. Each vertical coil is wound on a smooth, magnetically permeable core, in a repeatable multilayer precision stack of wire turns with only the wire turns themselves providing the support for maintaining the stack. The character and consistency of the display is not compromized.
The precision vertical coils of the deflection yoke of the invention lend themselves to manufacture on an automatic winding machine of conventional construction. An independently sequenced stepping operation of core advancement and wire flyer position readily permits positioning of the core with respect to the wire flyer and enables precision wire stacking. Multiple layers are built up in clusters on the core (with only wire turns providing support for the layers) rather than layer wrapping to the end and tracing back as in the art. This wire cluster arrangement reduces internal resonant currents.
The precision stack also enables precise alignment between the vertical and horizontal deflection coils. (Heretofore the sets of coils were positioned relative to each other in a separate operation in which their mutual interaction in an operating environment was observed.) In the preferred embodiment the liner has locating tabs and the cores have positioning notches. The completed vertical coils are just "dropped" into a unique position on the liner without the need for testing and adjustment. (The saddle horizontal coils are uniquely located on the underside of the liner.)